In the past, there have been several problems concerning the anodizing of aluminum, particularly hard coat anodizing which conventionally has required higher processing voltages. First, the voltage required to maintain current increases as the coating of aluminum oxide builds upon the aluminum part being anodized. The final voltage required for a 0.002 inch thick coating can exceed 70 votls DC (average), which results in a very large power consumption. Second, since the electrolyte in the anodizing tank must be maintained at a constant temperature, the heat generated by the relatively high power input must be removed by large, costly refrigeration equipment which is expensive to operate from an energy standpoint. Third, the current must be built up very slowly in order to condition the part for full current density. This typically results in a 20 minute start-up period where the tank is not coating at full speed, but instead is in a ramping stage. Fourth, thick coatings are difficult to produce because above a certain thickness, the average voltage required between anode and cathode is about 70 volts DC or more. At this voltage, the amount of power going through the part being coated often makes the coating unstable and may produce rapid dissolution and burning. Also, at this point the voltage is increasing exponentially while the coating thickness is increasing linearly, resulting in excessive power use for limited incremental increases in coating thickness. Finally, anodized coatings are difficult or impossible to produce in aluminum alloy containing copper as an alloying element using most conventional processes. Attempts to coat these alloys by such processes results in burning and dissolution of the metal.
There have been several attempts to solve these problems. Both electrical and chemical modifications have been tried. The electrical modifications have involved changing the waveform from the power supply, and the two major techniques may be called the pulsed DC technique and the AC over DC technique. In the pulsed DC technique the power supply is modified to produce regularly spaced DC current pulses, with no current flowing during the time between the pulses. It is believed that the periods of time between the pulses when no current is flowing allows the part to cool, and the electrolyte to rejuvenate. Systems employing this pulsed DC technique do decrease the ramp time required, thereby promoting process efficiency, and do make it easier to process copper bearing aluminum alloys. However, although the coatings can be applied more quickly using this technique the process requires higher current so that the cost-savings achieved by increase in processing speed is offset by the added cost of increased energy consumption. In the AC over DC technique, the power supply is modified to produce a small reverse current, or negative, pulse between each forward current, or positive, pulse. These systems have helped with all of the problems, except overall energy use. However, in order to operate then successfully, it is necessary to very closely monitor the amount of reverse current applied to the workpiece, because excessive reverse current is known to damage the anodic coating being formed on the part being anodized.
Several U.S. patents describe anodizing systems and processes which utilize large positive or forward current pulses and relatively small reverse current pulses. Such systems and processes may be viewed as one form of the AC over DC technique. The following U.S. patents are included in this group of patents:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,597,339 Newman et al. 3,975,254 Elco et al. 3,983,014 Newman et al. 4,517,059 Loch et al. ______________________________________
In the first three of these patents the anodizing system which is described therein used a silicon controlled rectifier (SCR) or comparable switching element connected to a transformer to generate controlled negative current pulses near the end of or in the middle of the negative-going portion of the AC waveform of the secondary of the transformer. In the fourth patent, namely U.S. Pat. No. 4,517,059, the negative current pulses are produced by connecting a negative polarity DC power supply between the part to be anodized and cathode of the system via a power drive in the form of a relay. Thus, in each of the prior art systems disclosed in these patents, the negative current flow through the part to be anodized is achieved by applying a source of negative voltage between the anode and cathode of the system.
One of the co-inventors for the present invention, working with others, developed a new power control apparatus for anodizing systems and processes which doese not require the use of a negative power supply. Instead, this new apparatus and process relies upon the retained charge present across the inherent capacitance of the anode and a part to be anodized and the electrolyte and cathode to discharge itself by providing a continuously connected shunt discharge means connected between the anode and cathode. This new system and process is described in U.S. patent application Ser. No. 943,510 filed Dec. 19, 1986 and entitled "LVA Anodizing Process, Apparatus and Product", the disclosure of which is hereby incorporated by reference herein. This appliation discloses that a resistive shunt discharge means and a pulsed DC power supply may be utilized to produce cyclic alternate charging and discharging cycles of the inherent capacitance formed between the anode and workpiece and the electrolyte and cathode. Moreover, the aforementioned application reports that the new apparatus and method provide significant benefits which include, among other things: (1) lower voltages being required to maintain the current needed to effect the required anodizing reactions in the bath; (2) the ability to easily form metal oxide coatings to thicknesses heretofore unachievable with conventional prior art systems; (3) the reduction of time required to anodize a part to given thickness; and (4) the ability to more closely control porosity and strength of the oxide coating produced by the anodizing process.
However, we have found that the disclosed continuous connection of a shunt discharge means permanently across the anode and cathode wastes electrical power unnecessarily, especially whenever the DC power supply system is producing positive current flow into the part to be anodized. We also realize that it would be beneficial to provide a switchable shunt discharge means, which discharge means could effectively be removed from the circuit whenever the DC power supply was attempting to provide positive current to the workpiece. With these thoughts in mind, we set out to devise a suitable fully automatic power control apparatus and method which would switch a shunt discharge means, such as a low ohmage power resistor, in and out of the overall anodizing process. Numerous problems were encountered, particularly with reliably determining under widely varying processing conditions when the positive current flow from power supply was acutally being turned on and off. Our efforts and testing over a period of months enabled us to refine our objectives to those stated below and develop the apparatus and method of the present invention described in detail below.
The principal object of the present invention is to provide an automatic power control apparatus and method for operating an aluminum anodizing system using a switchable shunt discharge means to discharge, quickly and immediately after the cessation of every charging cycle, the accumulated charge stored across the inherent capacitance existing between the anode and cathode of the system. Related objects of the present invention include: (1) fully automating this power control apparatus and method in a manner that avoids the need for expensive or complicated equipment for monitoring or controlling reverse control levels; (2) reliably detect the termination of a positive current flow to the workpiece under widely varying process conditions; (3) reliably detecting the beginning of positive current flow to the workpiece under widely varying process conditions; and (4) very quickly discharging at a optimum rate the built-up charge stored across the inherent capacitance.